1. Field of the Invention
The present invention relates to a transconductor, and more particularly, to a fully differential transconductor whose input and output are all differential signals, and an integrator and a filter circuit comprising the transconductor.
2. Description of the Related Art
Complex filters for use in Low-IF radio architecture play a role in efficiently eliminating an aliasing image signal in the same band in which desired waves are present, in the course of down conversion. By using the complex filter, an image eliminating filter having a high Q value is no longer required. Therefore, the complex filter is recognized as an important part for development of a one-chip radio system and a reduction in cost.
In recent years, there is a keen demand for low-power radio systems in terms of portability and the like. Therefore, it is also one of important challenges to achieve a low-power complex filter. Initially, a complex filter composed of an operational amplifier was proposed (see, for example, Japanese Patent No. 2988583). Later, a lower-power complex filter having a Gm-C structure (hereinafter also referred to as a Gm-C filter) was proposed (see, for example, Japanese Patent No. 3584893).
The Gm-C filter refers to a filter composed of a transconductor (Gm) which converts a voltage signal into a current signal and a capacitor (C) which integrates the current signal. The Gm-C filter has lesser linearity than a filter employing an operational amplifier, and therefore, often has a fully-differential structure. Therefore, a common-mode feedback circuit is required to stabilize a bias point (also referred to as a common-mode operating point) of a differential output.
In fully differential Gm-C filters, common-mode oscillation may occur due to the structure thereof. Therefore, it is considerably important to stabilize a common-mode potential in the fully differential Gm-C filter. Therefore, a relatively large common-mode feedback circuit is provided in conventional fully differential Gm-C filters. Particularly, when a complex filter is composed of a fully differential Gm-C filter, a common-mode potential becomes considerably unstable due to the structure of the complex filter, and therefore, a lager-scale common-mode feedback circuit is required. However, the large common-mode feedback circuit is responsible for an increase in chip area and power consumption. Also, a common-mode current in a signal line may be increased so as to stabilize the common-mode potential, however, this leads to a deterioration in noise characteristics.
FIG. 10 is a configuration of a conventional fully differential transconductor. The transconductor operates as a voltage-to-current conversion circuit which converts a differential voltage signal (Vin+−Vin−) into a differential current signal (Iout+−Iout−). The conventional transconductor has a common-mode gain of about 100-fold. Therefore, when the voltage signals Vin+ and Vin− are at the H level (or the L level) at respective input terminals, the current signals Iout+ and Iout− are at the L level (or the H level) at respective output terminals. In other words, the conventional transconductor has a property that the common-mode potential of the output differential signal changes largely with respect to a small change in the common-mode of the input voltage signal. Therefore, a common-mode feedback circuit (CMFB) for monitoring the voltage at the output terminal to adjust a bias for the differential current signal is required to stabilize the voltage of the output terminal. Further, a phase compensating capacitor is required to stabilize the common-mode feedback.
FIG. 11 illustrates a configuration of a complex filter composed of fully differential transconductors and capacitors. The complex filter can be divided into three main parts: an I filter 100, a Q filter 200, and a gyrator 300. The gyrator 300 plays a role in connecting the I filter 100 and the Q filter 200 in a quadrature phase relationship, and has an operation equivalent to that of a latch composed of cross-connected inverters when viewed in terms of a common-mode voltage. Specifically, positive feedback is performed so that, when the common-mode output terminal voltage of a transconductor Gm in any one of the I filter 100 and the Q filter 200 is at the H level (or the L level), the common-mode output terminal voltage of a transconductor Gm in the other is at the L level (or the H level). Further, a gm value of the gyrator portion is usually set to be several times or more larger than those of the other portions, positive feedback acts more strongly. As a result, the whole complex filter becomes unstable since. the common-mode voltage is positively fed back in a checkered pattern as indicated by H and L in FIG. 11.
Therefore, when a complex filter is composed of conventional fully differential transconductors, a common-mode feedback circuit which has a gain higher than that of the positive feedback in the gyrator and stabilizes the common-mode potential more quickly is essentially required. However, such a common-mode feedback circuit has large power consumption, and prevents the reduction of the power of a chip. Also, a noise generating factor, such as a current source or the like, is connected to a signal line, leading to a deterioration in noise characteristics. Also, a relatively large phase compensating capacitor is required to stabilize the common-mode feedback, leading to an increase in chip area.